This is what Track clubs are for - so athletes can develop outside the scholastic environment as well. That there are so few clubs in the US is rather sad, and is why there is so much consternation for athletes once their College eligibility is finished - they are either good enough to turn pro with a good contract or there is little else. This is why so many good athletes fall through the cracks.
I find it appalling that so few even try out (go to Junior nationals) for these Junior teams - XC is even less well attended. With the talent available, the US junior XC teams should be competitive with the Africans (even if they age-cheat), but you have to at least make it to the trials meet. That people like Baxter have never even tried to make the XC team says loads about the priority of her coaches. That Ephraimson, Cain, and Hasay (in the past) have done so is, in part, why they are the runners they are - just as Sheila Reid did for Canada. You get experience by trying to compete with the best in the world - the opportunity is there (you don't even have to pay for it once you make the team - unlike the Canadians), it just has to made more important than a state title, which is what the HS coaches care about.